1. Field of the Invention
The present invention generally relates to refrigeration systems and, more particularly, relates to heat transfer configurations for refrigeration systems including a plurality of evaporators and a compressor unit.
2. Related Art
In a typical refrigeration system, refrigerant circulates continuously through a closed circuit. The term "circuit", as used herein, refers to a physical apparatus whereas the term "cycle" as used herein refers to operation of a circuit, e.g., refrigerant cycles in a refrigeration circuit. The term "refrigerant", as used herein, refers to refrigerant in a liquid, vapor and/or gas form. Components of the closed circuit cause the refrigerant to undergo temperature/pressure changes. The temperature/pressure changes of the refrigerant result in energy transfer. Typical components of a refrigeration system include, for example, compressors, condensers, evaporators, control valves, and connecting piping. Details with regard to some known refrigeration systems are set forth in Baumeister et al., Standard Handbook for Mechanical Engineers, McGraw Hill Book Company, Eighth Edition, 1979, beginning at page 19-6.
Energy efficiency is one important factor in the implementation of refrigeration systems. Particularly, an ideal refrigeration system provides an ideal refrigeration effect. In practice, an actual refrigeration system provides an actual refrigeration effect less than the ideal refrigeration effect. The actual refrigeration effect provided varies from system to system.
Increased energy efficiency typically is achieved by utilizing more expensive and more efficient refrigeration system components, adding extra insulation adjacent to the area to be refrigerated, or by other costly additions. Increasing the energy efficiency of a refrigeration system therefore usually results in an increase in the cost of the system. It is desirable, of course, to increase the efficiency of a refrigeration system and minimize any increase in cost of the system.
In some apparatus utilizing refrigeration systems, more than one area is to be refrigerated, and at least one area requires more refrigeration than another area. A typical household refrigerator, which includes a freezer compartment and a fresh food compartment, is one example of such an apparatus. The freezer compartment preferably is maintained between -10.degree. Fahrenheit (F.) and +15.degree. F., and the fresh food compartment preferably is maintained between +33.degree. F. and +47.degree. F.
To meet these temperature requirements, a typical refrigeration system includes a compressor coupled to an evaporator disposed within the household refrigerator. The terms "coupled" and "connected" are used herein interchangeably. When two components are coupled or connected, this means that the components are linked, directly or indirectly in some manner in refrigerant flow relationship. Another component or other components can be intervening between coupled or connected components. For example, even though other components such as a pressure sensor or an expander are connected or coupled in the link between the compressor and evaporator, the compressor and evaporator are still coupled or connected.
Referring again to the refrigeration system for a typical household refrigerator, the evaporator is operated so that it is maintained at approximately -10.degree. F. (an actual range of approximately -30.degree. F. to 0.degree. F. typically is used) and air is blown across the coils of the evaporator. The flow of the evaporator-cooled air is controlled, for example, by barriers. A first portion of the evaporator-cooled air is directed to the freezer compartment and a second portion of the evaporator-cooled air is directed to the fresh food compartment. To cool a fresh food compartment, rather than utilizing evaporator-cooled air from an evaporator operating at -10.degree. F., it is possible to utilize an evaporator operating at, for example, +25.degree. F. (or a range of approximately +15.degree. F. to +32.degree. F.). The typical refrigeration system utilized in household refrigerators, therefore, produces its refrigeration effect by operating an evaporator at a temperature which is appropriate for the freezer compartment but lower than it needs to be for the fresh food compartment.
It is well-known that the energy required to maintain an evaporator at -10.degree. F. is greater than the energy required to maintain an evaporator at +25.degree. F. in a refrigerator. The typical household refrigerator therefore uses more energy to cool the fresh food compartment than is necessary. Using more energy than is necessary results in reduced energy efficiency.
The above referenced household refrigerator example is provided for illustrative purposes only. Many apparatus other than household refrigerators utilize refrigeration systems which include an evaporator operating at a temperature below a temperature at which the evaporator actually needs to operate.
Refrigeration systems which reduce energy use are described in commonly assigned U.S. Pat. Nos. 4,910,972 and 4,918,942. The patented systems utilize at least two evaporators and a plurality of compressors or a compressor having a plurality of stages. For example, in a dual, i.e., two, evaporator circuit for household refrigerators, a first evaporator operates at +25.degree. F. and a second evaporator operates at -10.degree. F. Air cooled by the first evaporator is utilized for the fresh food compartment and air cooled by the second evaporator is utilized for the freezer compartment. Utilizing the dual evaporator refrigeration system in a household refrigerator results in increased energy efficiency. Energy is conserved by operating the first evaporator at the temperature (e.g., +25.degree. F.) required for the fresh food compartment rather than operating an evaporator for the fresh food compartment at -10.degree. F. Other features of the patented systems also facilitate increased energy efficiencies.
To drive the plurality of evaporators in the refrigeration systems described in U.S. Pat. Nos. 4,910,972 and 4,918,942, and as mentioned above, a plurality of compressors or a compressor including a plurality of stages are utilized. Utilizing a plurality of compressors or utilizing a compressor having a plurality of stages results in increasing the cost of the refrigeration system over the cost, at least initially, of refrigeration systems utilizing one evaporator and one single stage compressor.
The refrigeration system described in U.S. patent application Ser. No. 07/612,290 provides improved energy efficiency achieved using a plurality of evaporators and minimizes, if not eliminates, the increase in cost associated with using a plurality of compressors or a compressor having a plurality of stages. Particularly, in one embodiment, the refrigeration system described in U.S. patent application Ser. No. 07/612,290 comprises a refrigerant flow control unit and a compressor unit. In the exemplification embodiment, the compressor unit is a single stage compressor. The refrigerant flow control unit is coupled to a plurality of input conduits. Each conduit, in the exemplification embodiment, has refrigerant disposed therein, and each respective refrigerant is at a respective pressure. For example, a first input to the control unit is a high pressure refrigerant and a second input to the control unit is a low pressure refrigerant. The outlet of the refrigerant flow control unit is coupled to the inlet of the compressor unit.
In operation, the respective refrigerants are provided as inputs to the control unit as described above, and the control unit provides that each respective refrigerant flows, alternately, to the compressor unit. The refrigerant flow timing, i.e., the length of time each input refrigerant is allowed to flow to the compressor unit, is determined on a straight timed basis or in accordance with measurable physical attributes, such as the respective pressures, temperatures, densities, and/or flow rates of the respective refrigerants.
In one circuit embodiment, when the freezer evaporator encounters thermal loads which are substantially below design load for example, some unevaporated liquid refrigerant is discharged from the freezer evaporator. The potential cooling capacity of the freezer evaporator, therefore, is decreased under these conditions, yet the amount of work required of the compressor unit is substantially unaffected.
Some of the lost cooling capacity is regained by disposing the conduit, i.e., the suction line, connected to the outlet of the freezer evaporator in a heat transfer arrangement with the conduit connected to the outlet of the condenser. Refrigerant liquid exiting the condenser is further subcooled as a result of the heat transfer arrangement thereby decreasing the enthalpy of the refrigerant before expansion in the fresh food evaporator. This heat transfer effectively shifts the specific cooling capacity, i.e., [(mass flow).times.(enthalpy change)], regain from the freezer evaporator to the fresh food evaporator.
It is well known, however, that the mechanical energy required to provide mass flow to the freezer evaporator is greater than the mechanical energy required to provide mass flow to the fresh food evaporator, i.e., more mechanical energy is required to operate an evaporator at a lower temperature. Although the above described heat transfer provides regain of cooling capacity, it would be most desirable if at least some of the cooling capacity regain is provided to the freezer evaporator, thereby decreasing the mechanical energy required to operate the freezer evaporator.
It is an object of the present invention to improve the energy efficiency of a refrigeration system which includes a single compressor unit coupled, directly or indirectly, to a plurality of evaporators.
Another object of the present invention is to provide regain of cooling capacity in an evaporator which operates at a low temperature in a refrigeration system.
Still another object of the present invention is to decrease the mechanical energy required to operate a refrigeration system having a plurality of evaporators.